Head engagement during visuomotor tracking is determined by postural demands and aging

Vision is important for various tasks, from visually tracking moving objects to maintaining balance. People obtain visual information through eye movements performed either alone or in combination with head movements. Even when isolated eye movements can accommodate the amplitude of the desired gaze shift, humans still perform head movements, as they provide additional sensory signals that can be integrated with retinal input resulting in improved gaze estimates. However, head movements also create mechanical torques and attenuate vestibular processing that could disturb balance. We, therefore, here examined whether head engagement is determined by postural requirements when performing a visual tracking task. Young participants visually tracked a target moving horizontally along different amplitudes, while they were seated, standing on a firm and an unstable surface. Our results showed stronger head engagement when standing than sitting, but no systematic differences were found between firm and unstable surfaces. To further explore the interplay between head engagement and postural demands, we conducted a second experiment where young and older participants performed a similar task, but now they were either allowed to move their head or instructed to limit their head movements. Both tracking accuracy and postural sway increased when engaging the head. When asked to limit head movements, both age groups engaged their head minimally, but head movements were more pronounced in more challenging postures. When allowed to move their head naturally, younger participants engaged their head more when standing than sitting, but older adults reduced their head movements with more demanding postures. We suggest that head movements in younger adults facilitate visual tracking, while limited head movements in older adults preserve balance.